Thanks Matt, that kid of yours is a natural!
I was up in my parents loft recently and found a box of some builds I did as a kid, they were rubbish compared to that RA-5c.
The panel lines on the La-7 were done with pastels. Scrape the pastels (I used dark browns and black) with a blade into a container. They scrape into dust. Then add a couple of drops of washing detergent and water. Paint the entire plane with this mix, and when its dry wipe it off with a damp piece of kitchen paper. The pastel wash remains in the panel-lines.
**Give the whole plane a coat of Klear/Future before doing this, if you don't it will darken the paint (if it's a matt paint).
I normally give it the pastel wash after the decals are on, but I got a bit ahead of myself with the La-7 and gave it a panel wash before the decals went on.
I discovered another way of weathering with panel wash by mistake a while ago. I painted a A6M5 zero's topside green, added the decals, then decided to really weather it with a lot of metal showing thru. I went along the panel lines with a sponge dipped in silver enamel. I then gave it a coat of Klear/Future, and gave it a panel wash. I wasn't happy with the paint chipping, it was WAY over the top. So i used the sponge again with some of the green paint on it and went over the chipping I had done to tone it down a bit. I also went over the hinomaru's with a bit of red paint on the sponge. When that was dry I gave it a pastel wash, forgetting I hadn't given it a coat of Klear 1st and it darkened the green and red paint I'd applied with the sponge. But I think the mistake I made actually worked!